Recently, as thermoelectric power stations become larger in size there is an increasing demand for a boiler capable of being operated at high temperatures and pressures. In general, when steam having a temperature exceeding 550.degree. C. is to be generated in a boiler, such a boiler should be made not of a conventional 21/4Cr-1Mo steel, but of a high quality steel, such as a 18-8 stainless steel, having a remarkably enhanced resistance to oxidation and mechanical strength at an elevated temperature, as compared with the 21/4-Cr-1Mo steel.
However, stainless steel is far more expensive than a low-alloyed steel. Therefore, the use of stainless steel results in an increase in the cost of the boiler production. From such an economical point of view, usually, the operational temperature of the boiler is limited so that the temperature of steam generated from the boiler does not exceed 550.degree. C. Under these circumstances, in order to enhance the boiler efficiency, a super critical boiler, which is operated at a pressure higher than the critical pressure of water, is used.
On the other hand, for the past thirty to forty years, various research has been made to develop a middle-graded steel between the 21/4-Cr-1Mo steel and the austenitic stainless steel. For example, attention was paid to boiler steel tubes having an intermediate content of chromium between that in the 21/4Cr-1Mo steel and that in the stainless steel, such as a 5Cr, 9Cr or 12Cr tube. Although this type of boiler steel tube has a satisfactory mechanical properties at an elevated temperature, the weldability and formability thereof are inferior. Therefore, in order to improve the weldability and formability of the steel tube, extensive studies were made. However, no satisfactory result was obtained. Therefore, the use of such a steel is necessarily accompanied by a significant low efficiency of construction for the boiler. In practice, the above-mentioned steel tube has not been used in the world except for a small area in Europe.
Under these circumstances, there is a strong desire for the development of an inexpensive heat-resisting steel having an intermediate creep rupture strength between that of the 21/4Cr-1Mo steel and that of the austenitic stainless steel.
In view of the above, the inventors of the present invention made extensive studies to develop a novel steel having an enhanced weldability and formability and a remarkably enhanced creep rupture strength, over those of the conventional boiler steel materials. As a result of this, the inventors of the present invention found that a reduced content of carbon is liable to reduce the toughness of the resultant steel during the use thereof. Therefore, in consideration of the balance between the contents of the alloying elements to be contained, the inventors of the present invention created a new type of ferritic steel.